1. Field of the Invention
The present invention relates to a permanent-magnet electric rotating machine, and particularly relates to a two-phase permanent-magnet electric rotating machine low in cost and adapted for use in office automation appliances such as copying machines, facsimiles, or the like, which require relatively accurate positioning functions, or the like, with low noise and low vibration.
2. Description of Prior Art
As conventional electric rotating machines for use in office automation appliances which do not require high accurate positioning functions specially, two-phase permanent-magnet electric rotating machines which can be produced at a low cost are widely used of the two-phase permanent-magnet electric rotating machines, a hybrid type permanent-magnet electric rotating machine is generally used because the characteristic of a small step angle is obtained with high accuracy and high torque.
FIGS. 8A and 8B conceptually show an example of the main structure of a conventional two-phase four-pole hybrid type permanent-magnet electric rotating machine. In FIGS. 8A and 8B, the details of a housing, rotor shaft bearings and windings and lead wires, and so on, are not shown. FIG. 8A is a front view of the electric rotating machine, and FIG. 8B is a section taken along the line VIIIB--VIIIB in FIG. 8A.
In FIGS. 8A and 8B, the reference numeral 1' designates an annular yoke portion of a stator iron core. Four main poles 1'-1 to 1'-4 are formed at circumferentially equal pitches so as to project centripetally toward the inside of the yoke 1'. Excitation windings 2'-1 to 2'-4 for performing magnetization in predetermined timing are provided on the four main poles 1'-1 to 1'-4, respectively.
Magnetic teeth 1'a of the number corresponding to the structural characteristic of the electric rotating machine are formed at circumferentially equal intervals on each of the main poles 1'-1 to 1'-4.
A rotating shaft 3.sup.n is rotatably supported by shaft bearings (not shown) provided in a center portion of a housing (not shown). An axially magnetized permanent magnet 4' is fixedly provided on the rotating shaft 3'. The permanent magnet 4' is sandwiched between two disc-like rotor magnetic poles 5'A and 5'B. Magnetic teeth 5'a are formed on the outer circumference of each of the rotor magnetic poles 5'A and 5'B so as to be arranged with a shape and pitch corresponding to the shape and pitch of the magnetic teeth 1'a formed on the stator main poles. The magnetic teeth 5'a of the first rotor magnetic pole 5'A are coupled with the magnetic teeth 5'a of the second rotor magnetic pole 5'B with a relative displacement in the direction of rotation by a half pitch.
In the electric rotating machine configured as described above, the respective magnetic teeth 1'a of the stator are magnetized successively in the direction of rotation by supply of an electric current to the stator windings 2'-1 to 2'-4 in a predetermined sequence and in a predetermined direction.
In FIGS. 8A and 8B, the symbol N (north) written in the stator main pole 1'-1 and the symbol S (south) written in the stator main pole 1'-3 show magnetic polarities generated when excitation currents are made to pass through the windings 2'-1 and 2'-3 in certain timing, respectively. The symbols N (north) and S (south) written in the magnetic teeth 5'a in FIG. 8A and written in the first and second rotor magnetic poles 5'A and 5'B in FIG. 8B show an example of polarities of rotor magnetic poles magnetized by the permanent magnet 4', respectively.
That is, in the timing shown in FIGS. 8A and 8B, the magnetic teeth 1'a of the main pole 1'-1 and the magnetic teeth 5'a of the first rotor magnetic pole 5'A are attracted to each other, and the magnetic teeth 1'a of the main pole 1'-3 and the magnetic teeth 5'a of the second rotor magnetic pole 5'B are attracted to each other.
By the aforementioned action, the rotor is rotated or stopped by means of the interaction between the stator magnetic teeth 1'a magnetized in the direction of rotation and the rotor magnetic teeth 5'a magnetized by the permanent magnet 4'.
In the aforementioned conventional two-phase hybrid type permanent-magnet electric rotating machine, the first rotor magnetic pole 5'A and the second rotor magnetic pole 5'B are attracted upward and downward, respectively, in the drawing as represented by the arrows in the excitation timing shown in FIG. 8B, so that imbalance force acts on the rotating shaft 3' as shown in FIG. 8B. Accordingly, as the excitation of the respective main poles of the stator rotates, the rotor vibrates to thereby generate noise while it is rotating, because there is a clearance between the rotating shaft 3' and the outer or inner wheel of the not-shown shaft bearings.
That is, of the two-phase permanent-magnet electric rotating machines, such a generally used hybrid type electric rotating machine is low in cost and high in accuracy but has a problem in that vibration as well as noise are large.
An object of the present invention is to solve the aforementioned problems in the prior art, that is, to provide a two-phase permanent-magnet electric rotating machine which is low in vibration as well as noise and which can be produced at a low cost and in a small size.